By endowing polymeric resins with nanometer-scale structure and order, researchers have prepared catalysts that outperform their amorphous resin counterparts. The synthesis methods used to prepare the materials can be adjusted to prepare custom products, which may lead to improvements in commercial solid acid catalysts.
Substituting acidic solids for the liquid mineral acids used to catalyze industrial-scale reactions offers a number of benefits. Using solids avoids some of the safety and environmental hazards associated with caustic liquids. And replacing liquids with solids simplifies the separation of catalyst from product and may lead to reduced equipment corrosion and to engineering designs with improved efficiency.
Motivated by these and other benefits, scientists have developed acidic forms of zeolites and polymeric resins, and chemical manufacturers have put the materials to use. Resins based on sulfonic acid or perfluorosulfonic acid--such as Amberlyst, made by Rohm and Haas, and DuPont's Nafion--are used in several industrial processes. The list includes etherification of olefins with alcohols, as used, for example, to make methyl tert-butyl ether; alkylation and condensation reactions; dehydration of alcohols; and other processes.
The list of applications and the scale at which solid acid catalysts are used commercially could grow significantly if scientists could devise a scheme for making materials with the versatility and processing convenience of polymeric resins (amorphous films) and the selectivity and nanoscale structure of crystalline zeolites. Progress in that direction has been slow because, among other reasons, some of the candidate materials lack the requisite catalytic properties.
But now, by devising a novel liquid-crystalline system with acidic character and locking the self-assembling structure in place via polymerization, researchers at the University of Colorado, Boulder, have prepared a strongly acidic resin with well-defined structure and nanosized pores that functions effectively as a heterogeneous catalyst [J. Am. Chem. Soc., 126, 1616 (2004)].
"We had been trying for about six years to make an organic analog of an acidic zeolite with well-defined pores and channels," says Colorado's Douglas L. Gin, who led the study. Gin, an associate professor of chemistry and chemical engineering, explains that some years ago his research group succeeded in demonstrating base catalysis using materials that the group prepared from liquid-crystal systems.
"But something about strong acid groups messes up the system's ability to organize into well-defined liquid-crystalline phases," he says. That difficulty prevented researchers from incorporating catalytically useful sulfonic acid groups into liquid crystals.
Just recently, though, Gin's group hit upon a solution. "We found that there's something special about amide hydrogen bonding that enhances formation of liquid-crystalline phases," he notes. So Gin and coworkers Yanjie Xu and Weiqiang Gu prepared two types of monomers that feature amide linkages adjacent to the head groups. One of the monomers contains sulfonic acid groups--to catalyze reactions. The other molecule, a weak acid structure-directing agent, contains carboxylic acid head groups.
Exploiting hydrogen bonding between amide groups of neighboring molecules to drive self-assembly of a blend of monomers and then pinning the liquid crystal's structure in place via polymerization, the Colorado group succeeded in preparing an acidic resin featuring hexagonal arrays of channels with nanometer-sized openings. Then the group used the new material to catalyze esterification of benzyl alcohol with 1-hexanoic acid. They found that their resin was nearly as active as samples of Nafion and Amberlyst but more than 10 times as selective as the commercial resins in forming the ester product relative to dibenzyl ether, an unwanted side product.
"The work is not only interesting academically but also has relevance in the industrial sector, where environmental and economic benefits are of prime importance," remarks SonBinh T. Nguyen, an associate chemistry professor at Northwestern University. Nguyen emphasizes that the method is unique in that it can be used to design solid acids that combine activity, selectivity, and tunability.
Tunability is where Gin is focusing next. Recently, his group demonstrated that the nanostructured resins could be formed from weak and strong acid monomers blended in a range of ratios. The task at hand is to study the way that adjusting the acid strength affects catalysis.-- MITCH JACOBY